A Study of Nemtabrutinib Plus Venetoclax vs Venetoclax + Rituximab (VR) in Second-line (2L) + Relapsed/Refractory (R/R) Chronic Lymphocytic Leukemia/Small Lymphocytic Lymphoma (CLL/SLL) (MK-1026-010/BELLWAVE-010).

Inclusion Criteria:

• Confirmed diagnosis of chronic lymphocytic leukemia/small lymphocytic lymphoma (CLL/SLL) and active disease clearly documented to initiate therapy

• Deletion (Del) (17p) status, tumor protein 53 (TP53) mutation status, andimmunoglobulin heavy chain gene (IGHV) mutation status results required beforerandomization for Part 2 participants only

• Relapsed or refractory to at least 1 prior available therapy

• Have at least 1 marker of disease burden

• Has an Eastern Cooperative Oncology Group (ECOG) performance status of 0 to 2 within 7 days before randomization

• Has a life expectancy of at least 3 months

• Has the ability to swallow and retain oral medication

• Participants who are hepatitis B surface antigen (HBsAg) positive are eligible ifthey have received hepatitis B virus (HBV) antiviral therapy for at least 4 weeksand have undetectable HBV deoxyribonucleic acid (DNA) viral load beforerandomization

• Participants with history of hepatitis C virus (HCV) infection are eligible if HCVribonucleic acid (RNA) viral load is undetectable at screening

• Participants with human immunodeficiency virus (HIV) who meet ALL eligibilitycriteria

• Participants with adequate organ function with specimens collected within 7 daysbefore the start of study intervention

• If capable of producing sperm, participant agrees to eliminate Nemtabrutinib: 12days, Venetoclax: 1 month (30 days), Rituximab (rituximab biosimilar): notapplicable; abstains from penile-vaginal intercourse as their preferred and usuallifestyle; OR uses prescribed contraception

• Participant assigned female sex at birth are eligible to participate if not pregnantor breastfeeding and are not a person of childbearing potential (POCBP) OR is aPOCBP and uses a contraceptive method that is highly effective, has a negativehighly sensitive pregnancy test, and abstains from breastfeeding

Exclusion Criteria:

• Has an active hepatitis B virus/ hepatitis C virus (HBV/HCV) infection

• Has gastrointestinal (GI) dysfunction that may affect drug absorption

• Has a known additional malignancy that is progressing or has required activetreatment within the past 2 years

• Has diagnosis of Richter Transformation or active central nervous system (CNS)involvement by CLL/SLL

• Has an active infection requiring systemic therapy, such as intravenous (IV)antibiotics, during screening

• HIV-infected participants with a history of Kaposi's sarcoma and/or MulticentricCastleman's Disease and/or acquired immune deficiency syndrome (AIDS)-definingopportunistic infection in the past 12 months before screening

• Clinically significant cardiovascular disease

• Has a known allergy/sensitivity to nemtabrutinib or contraindication tovenetoclax/rituximab (or rituximab biosimilar), or any of the excipients

• Has history of severe bleeding disorders (eg, hemophilia)

• Has received prior systemic anticancer therapy within 5 half-lives or 4 weeks (ifprior therapy was a monoclonal antibody) before randomization

• Has received prior B-cell lymphoma 2 inhibitor(s) (BCL2i) within ≤ 12 months beforerandomization or has received prior radiotherapy within 2 weeks of start of studyintervention, or radiation related toxicities, requiring corticosteroids

• Is currently being treated with p-glycoprotein (P-gp) substrates with a narrowtherapeutic index, cytochrome P450 3A (CYP3A) strong or moderate inducers or CYP3Astrong inhibitors.

• Has received a live or live attenuated vaccine within 30 days before the first doseof study intervention

• Has received an investigational agent or has used an investigational device within 4weeks before study intervention administration

• Has a known psychiatric or substance use disorder that would interfere with theparticipant's ability to cooperate with the requirements of the study

• Participants who have not adequately recovered from major surgery or have ongoingsurgical complications